1. Field of the Invention
The present invention relates to excimer laser equipment which is one kind of gas laser equipment, in particular to a rare gas-halide excimer laser equipment using rare gases and halogens as laser media.
2. Description of the Prior Art
A gas laser has advantages in that (1) a stable continuous oscillation is easily achieved; (2) oscillated lights are superior in monochromaticity, interferability, directivity and collectivity; (3) the oscillation central frequency is not dependent upon a circumferential temperature; (4) the oscillation wave length is easily stabilized; (5) several thousands of oscillation wave lengths are obtained from a vacuum ultraviolet range to a mm-wave range; (6) although usually an output being small in comparison with the size of the equipment, a volume of laser media can be increased, whereby an output can be increased; and the like.
An excimer laser, which is one kind of such gas lasers, is a high-efficiency and large-output shortwave laser which can be used as a power source for use in laser nuclear fusion, for use in an optical communication, and a light source for use in a communication in the sea or between the sea and the sky, for example the sea and artificial satellites.
It is, however, usually known that in a rare gas-halide excimer laser a laser pulse energy is gradually reduced with a repetition of oscillation, for example in a XeCl excimer laser using xenon (Xe) as a rare gas and chlorine (Cl) as a halogen, a laser pulse energy is reduced to about a half of the initial value after about 10.sup.6 times of pulse oscillation.
The main reason for the above described phenomenon is that halogen, which is one component of laser media, is described by the reaction upon materials, of which a laser housing is made, or spattered metals produced by an exciting electric discharge.
However, we do not have a simple apparatus or method for determining halogens, so that it has not been easy to determine the reduction of halogens contained in laser media. Accordingly, it has been difficult to determine a quantity of halogen to be added for making up for the loss of halogens, whereby when a laser pulse energy was reduced, all of laser media, which had been used in laser equipment, had to be replaced by fresh laser media or a part of the laser media had to be replaced by fresh laser media at an appointed time.
The conventional methods will be concretely described below with reference to FIG. 5 which schematically shows the conventional excimer laser equipment.
In FIG. 5, reference numeral 1 designates a laser housing, 5 designates a buffer diluted halogen gas-bomb, 9 designates a secondary pressure regulator, 16 designates a control system, 57 designates a laser beam, 58 designates a beam splitter, 59 designates a part of the laser beam, 60 designates a laser-output detector, and 61 designates a control system of flow rate.
In such conventional equipment, at first a part of the laser beam 57 is taken out as the part 59 of the laser beam by the beam splitter 58 and an output of the part 59 of the laser beam is measured by the laser-output detector 60. If the resulting measured value is reduced below the appointed value, the control system 16 actuates the control system of flow rate 61 to supply halogens from the buffer diluted halogen gas-bomb 5 through the secondary pressure regulator 9 until the laser-output is recovered.
However, since the change of the laser-output is influenced by a change of power source voltage, the instability of exciting electric discharge, the contamination of a laser pulse-taking out window, and the like, in addition to the above described reduction of the concentration of halogens, it is obvious that the change of the laser-output does not directly correspond to the change of the concentration of halogens. For example, also when the laser pulse energy is reduced due to the reduction of power source voltage, the control system supplies halogens on the assumption that the concentration of halogens was reduced, so the still further increased concentration of halogens is indicated. In this case, since halogens destabilize the electrical discharge, vicious cycles, in which the destabilization of exciting electrical discharge leads to the further reduction of the laser-output, whereby supplying a further excessive amount of halogens, are repeated. And, at last, not only the laser-oscillation itself is stopped but also the exciting electrical discharge is transferred to an arc, so that the problems such as an electrode being damaged, expensive halogens and rare gases being wastefully consumed, whereby increased operating cost, and the like, have occurred.